AFTER PERSUADING Frank Rizzo to take a lie-detector test, editing the Daily News for 20 years and giving voice to the city's ethical conscience for an additional 10, Zack Stalberg will soon go from watchdog to cowboy.

Stalberg announced yesterday that he is stepping down as president of the Committee of Seventy, the 110-year-old good-government nonprofit, and moving to Lamy, N.M., with his wife, Deb Lock Stalberg, and their horse, Mr. B.

"We haven't broken the news to him yet," Stalberg, 67, said of Mr. B. "Exactly what I'll be doing, I'm not sure, but I love horses and Western life and hope part of it will be outdoors. I've been behind a desk for too long."

Stalberg's legacy in Philadelphia was cemented in 1973, when he was a City Hall reporter for the Daily News. Grasping for a story on a slow afternoon, Stalberg persuaded then-Mayor Rizzo to take a polygraph test to prove his case in a he-said/he-said spat with the city Democratic Party chairman.

Rizzo failed, and the classic headline that resulted - "RIZZO LIED, TESTS SHOW" - became part of his legacy, as well.

Fittingly, the episode has resurfaced at the end of Stalberg's career. In this month's Philadelphia magazine, Simon van Zuylen-Wood wrote about Frank S. Rizzo, the former councilman and son of the former mayor who wants to run for mayor next year.

In the story, political consultant Neil Oxman offered to pay Rizzo $10,000 to sit for a polygraph and prove that he isn't running a faux campaign just to improve the odds for another candidate, state Sen. Anthony Hardy Williams. Junior passed, Oxman paid, and once again, Stalberg had a role.

"I may have suggested it actually," Stalberg said. "I was bulls---ting with Simon, and either I said it to him or he said it to me. I thought it was hilarious."

Stalberg said that although he respects Rizzo, he hopes the 71-year-old will follow his lead and take a bow.

"The politicians, too, have to know when to move off the stage," he said. "There are in almost every sector younger, motivated, change-oriented people lining up, and they're having trouble breaking through."

Asked who he hopes will run for mayor, Stalberg said he wants a political outsider to get in.

"Generally speaking, I wish it were a more impressive field than it seems to be at the moment," he said. "I don't hear any serious talk of an unorthodox candidate, and to me that's worrisome. I'm not sure we're going to get fresh answers or bold conduct from career politicians."

Seventy's board, composed of local leaders from the business, law, labor and nonprofit sectors, has not yet decided who will replace Stalberg but indicated that it will be someone who will continue the aggressive style that has marked his tenure.

The group "went through a few decades of being asleep at the wheel and then the bug was found in John Street's office," Stalberg said, referencing the FBI investigation of the last mayoral administration. "By now I think it's definitely in the DNA of the organization, and it's a different board and a board that really supports the idea of a much more aggressive watchdog."

Aside from his service in the Army during the Vietnam War, Stalberg has lived in the Philly area his entire life. A fan of Western movies - "Shane" is his favorite - he said the move to New Mexico will allow him to fulfill a longtime dream.

"It's been a great ride, and I've had a lot of fun. . . . I grew up here, of course," he said. "But there's something about the myth of the West and the romanticism of the West."